SwiftUI: Tips on inspecting/debugging a view? Authentication input presents mysterious transparent box

I have a very simple example project (one file below) which demonstrates an issue: when I start typing into either of the fields, a mysterious, transparent box appears below and extends outside the window bounds. This occurs even in a SwiftUI preview, as shown here:

How can I debug this, to determine what is creating that mysterious box? It seems to only occur when there is both TextField and SecureTextField, so I'm wondering if it's some kind of password assistant, or something like that. I'd like to find a way to track it down, and ultimately get rid of it, but I have no idea where to even begin looking.

import SwiftUI

@main
struct TestApp: App {
    var body: some Scene { WindowGroup { ContentView() } }
}

struct ContentView: View {
    @State var email: String = ""
    @State var password: String = ""

    var body: some View {
        VStack {
            TextField("Email Address", text: $email)
            SecureField("******************", text: $password)
        }.padding().frame(minWidth:150, minHeight: 300)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}